Laughing gas (nitrous oxide), commonly used as an anaesthetic during surgery, increases the risk of postoperative complications, newspapers reported. The Guardian explained that those patients who did not receive nitrous oxide during surgery were “50% less likely to contract pneumonia and 20-30% less likely to suffer fever or wound infection after surgery”.

The Daily Mail reported that people given laughing gas were “three times more likely to suffer a heart attack and five times more likely to have died in the month after surgery”. However, the paper goes on to say that the low number of events meant that this could have occurred simply by chance.

The report is based on a trial where people undergoing major surgery (lasting at least two hours) were spilt into two groups, with one given anaesthetic that contained nitrous oxide and the other given no nitrous oxide. Information on the outcome of the surgery, the rates of complications and the patients’ experiences of nausea and vomiting was collected for both groups and compared. The researchers concluded that the results raised questions about the routine use of nitrous oxide.

Where did the story come from?

Paul Myles and colleagues at Alfred Hospital in Melbourne, Australia, conducted this research. There is no report of funding received for the research. It was published in the medical journal, Anesthesiology .

What kind of scientific study was this?

The researchers enrolled 2,050 adults (over 18 years old) who were scheduled to undergo major surgery (except heart or chest surgery). Participants from 19 centres around the world were included and were randomly assigned to the type of anaesthetic they would receive.

The main point of the study was to see whether duration of hospital stay differed between the groups, although researchers also collected information on postoperative outcomes in the 30 days following surgery.

What were the results of the study?

The researchers found there to be no difference between groups in the duration of stay in hospital. However, they found that people who had the anaesthetic without nitrous oxide had a lower risk of major complications in the 30 days after surgery. This included pneumonia, wound infection, stroke, death, awareness during surgery, and venous thromboembolism. This group also experienced less severe nausea and vomiting in the 24 hours after surgery than the group who received anaesthetic containing nitrous oxide.

What interpretations did the researchers draw from these results?

The researchers conclude that because of the observed reduction in the risk of major postoperative complications with anaesthetic that does not contain nitrous oxide, “the routine use of nitrous oxide in patients undergoing major surgery should be questioned”.

What does the NHS Knowledge Service make of this study?

is a well-conducted trial that highlights an important area of current practice. Some aspects are still unclear and further study would help to establish whether anaesthesia with nitrous oxide is associated with adverse outcomes:

From this study it isn’t possible to determine whether it was the nitrous oxide in the anaethetic that was harmful or whether alternative treatment – a high concentration of oxygen – was beneficial. Though the researchers tried to establish this, there was not enough data available to draw conclusions. However, as they sensibly point out, whether the study was uncovering harms of one anaesthetic or additional benefits of the other is ”immaterial in a practical sense”. If a particular regimen results in better outcomes than another, its use should be considered.

Importantly, as highlighted by the authors, participants in this study were undergoing major surgery (of at least two hours' duration). The implications of these findings when using nitrous oxide in minor surgical procedures is yet to be established.

Additionally, it is also important not to draw any conclusions from these results on the effects of nitrous oxide use in childbirth (commonly known as “gas and air”). One of the news articles highlights its regular use for this indication (where slightly different gas concentrations to those used in surgery are used to provide pain relief, rather than to maintain anaesthesia); however this use has not been investigated by this research.

Further research in this area could shed light on some of these issues.